A methanation reaction comprises a catalytic reaction of hydrogen with carbon monoxide to produce methane. This methane is sometimes also referred to as synthetic natural gas (SNG) and can be used as substitute gas for natural gas. In areas where there is little natural gas available, other sources of energy, such as coal or petroleum coke, may be partially oxidized in a gasification process to produce a gas comprising hydrogen and carbon monoxide. Such a gas comprising hydrogen and carbon monoxide is sometimes also referred to as synthesis gas. The synthesis gas can subsequently be used to produce synthetic natural gas (SNG) in a methanation process.
The methanation reaction proceeds, in the presence of a suitable methanation catalyst, in accordance with the following equations:CO+3H2=CH4+H2O+heat  (1)CO2+4H2=CH4+2H2O+heat  (2).
The water formed during the reaction can, depending on the catalyst, temperature and concentrations present, subsequently react in-situ with carbon monoxide in a water-gas shift reaction in accordance with the following equation:CO+H2O=CO2+H2+heat  (3)
Reaction (1) is considered the main reaction and reactions (2) and (3) are considered to be side reactions. All the reactions are exothermic.
The methanation reactions are reversible and an increasing temperature will tend to shift the equilibrium towards a lower yield. To control the temperature, the methanation reaction can be carried out in one or more internally cooled reactors, where the reactants can be cooled by a coolant.
U.S. Pat. No. 4,839,391 describes a one-stage process for the methanation of synthesis gas to generate methane and superheated steam. The methanation reactor comprises a catalyst bed with different temperature regions, through which a cooling system passes. A cooling medium, i.e. water, flows through the cooling system countercurrently to the flow of synthesis gas through the catalyst bed. In the process, the synthesis gas successively flows through an inflow region, a hot spot region and an outward gas flow region. Cooling water is converted to steam by heat transfer in the outward gas flow region and hot spot region of the reactor, also referred to as vaporizer. Subsequently the steam is being superheated in the hot spot region of the reactor, also referred to as superheater. An external heat exchanger is used to preheat the water before entering the reactor against the effluent methane/synthesis gas mixture from the reactor. In operation example 8, synthesis gas flows into a methanation reactor at a pressure of 50 bars and a temperature of 280° C. and after methanation of the synthesis gas in the methanation reactor a product gas is obtained at a pressure of 47 bars and a temperature of 350° C. Preheated cooling water of 260° C. is introduced into a steam chamber, where it is brought up to a temperature of 310° C. at a pressure of 100 bars and then introduced into the vaporizer (boiler), in which it is converted to steam at a vaporization temperature of 311° C. (the boiling temperature of water at 100 bars is about 311° C.). The steam produced is led back into the steam chamber and then flows into the superheater where it is superheated to 500° C. at 100 bars.
A disadvantage of the process of U.S. Pat. No. 4,839,391 is the high exit temperature of the product gas, which limits the conversion in the last part of the methanation reactor.
U.S. Pat. No. 4,431,751 describes a method for producing superheated steam with the heat of catalytic methanation of a synthesis gas containing carbon monoxide, carbon dioxide and hydrogen. The process passes a gas stream first through a first internally water-cooled reactor, subsequently through an adiabatic reactor and a subsequent heat exchanger and finally through a second internally water-cooled reactor. The internally water-cooled reactors comprise a cooling system disposed within a catalyst bed through which water flows. As catalyst a nickel-containing catalyst is used. Water is passed in succession, first through the cooling system of the second internally water-cooled reactor for preheating thereof to a temperature approximating the saturated steam temperature, thereafter into the first internally water-cooled reactor for conversion into saturated steam and subsequently to the heat exchanger following the adiabatic reactor for superheating. In the exemplified process the second internally water-cooled reactor is set to operate at a gas entrance temperature between 250° C. and 350° C., specifically 300° C., at an average pressure of 37.5 bar and the product gas flowing out of the second internally water-cooled reactor is 300° C. In the example the water is brought up to a pressure of 110 bar and preheated up to 160° C. before entering the cooling system of the second internally water-cooled reactor (the boiling temperature of water at 110 bars is about 318° C.). A disadvantage of the process of U.S. Pat. No. 4,431,751 is that the very low entrance temperature of the water in the second internally water-cooled reactor, especially in combination with the cooling system used, can lead to cold spots in the catalyst bed which are expected to result deactivation of the nickel catalyst due to the formation of nickel carbonyl. In addition the process of U.S. Pat. No. 4,431,751 requires an adiabatic reactor to allow the steam to become superheated.
It would be an advancement in the art to provide a process for the production of methane wherein the above disadvantages are avoided.